Many motile prokaryotic and eukaryotic cells employ actin-based mechanisms to achieve movement. In particular, the bacteria Listeria monocytogenes move by assembling an actin filament-dense comet tail that propels them inside the cytoplasm of their host cells. Eukaryotic cells assemble actin networks that create membrane projections (lamellipodia,filopodia,etc.) that may drive whole-cell movement. Since quantitative cell motility studies have mainly analyzed the parameter of speed to evaluate molecular changes, this project seeks to explore molecular contributions that allow moving cells to change direction and achieve curvature in their trajectories using a systematic and quantitative approach. By analyzing protein distribution and actin network architecture in relation to Listeria monocytogenes movement in cell lines and during neutrophil chemotaxis and chemokinesis,we will specifically: 1. Characterize the molecular variations underlying Listeria monocytogenes directional motility 2. Determine the mechanism of action of molecules involved in bacterium-comet tail interactions influencing directional motility of Listeria monocytogenes 3. Characterize directional motility in neutrophils and begin analysis of molecular contributions by VASP (Vasodilator-stimulated phosphoprotein).